1. Field of the Invention
This application relates generally to sensor systems, and more particularly to optical-fiber-compatible sensor systems.
2. Description of the Related Art
Fiber-optic acoustic sensors have been extensively researched since their first emergence in the 1970s (see, e.g. J. H. Cole, R. L. Johnson, and P. G. Bhuta, “Fiber-optic detection of sound,” J. Acoust. Soc. Am. Vol. 62, 1136 (1977); J. A. Bucaro, H. D. Dardy, and E. F. Carome, “Fiber-optic hydrophone,” J. Acoust. Soc. Am. Vol. 62, 1302 (1977)), largely because of their multiple advantages over conventional acoustic sensors. These advantages include small size and weight, ability to operate in chemically and/or electrically harsh environments, ease of multiplexing large numbers of sensors together, and compatibility with fiber-optic networks for data transport and processing.
Various forms of biological, chemical, and mechanical sensors (such as acoustic or pressure sensors) that can be addressed optically at the end of an optical fiber can be useful for medical and security applications. The very small size (e.g. 125 μm diameter) of these sensors, can be used for example to penetrate tissue or veins, or to deploy in places where small size is crucial. Also, optical devices such as filters, mirrors, and polarizers at a fiber end can be very useful in fiber communication applications.
In several key applications, such as undersea oil exploration and smart wells, the demand for more sensitive and more compact fiber sensors has been a strong drive behind recent research efforts. A current limitation of acoustic fiber sensors is that in order to be highly sensitive, they require a long length of fiber, which makes them bulky and poorly to non-responsive to frequencies above a few hundred Hz.
Various fiber optic sensor systems have been previously disclosed that provide acoustic pressure measurements based on the relative displacements of the two mirrors of a Fabry-Perot interferometric cavity. See, e.g. M. Yu et al., “Acoustic Measurements Using a Fiber Optic Sensor System,” J. Intelligent Mat'l Systems and Structures, vol. 14, pages 409-414 (July 2003); K. Totsu et al., “Ultra-Miniature Fiber-Optic Pressure Sensor Using White Light Interferometry,” J. Micromech. Microeng., vol. 15, pages 71-75 (2005); W. B. Spillman, Jr. et al., “Moving Fiber-Optic Hydrophone,” Optics Lett., vol. 5, no. 1, pages 30-31 (January 1980); K. Kardirvel et al., “Design and Characterization of MEMS Optical Microphone for Aeroacoustic Measurement,” 42nd AIAA Aerospace Sciences Meeting and Exhibit, 5-8 Jan. 2004, Reno, Nev.; J. A. Bucaro et al., “Miniature, High Performance, Low-Cost Fiber Optic Microphone,” J. Acoust. Soc. Am., vol. 118, no. 3, part 1, pages 1406-1413 (September 2005); T. K. Gangopadhyay et al., “Modeling and Analysis of an Extrinsic Fabry-Perot Interferometer Cavity,” Appl. Optics, vol. 44, no. 16, pages 312-3196 (1 Jun. 2005); and P. J. Kuzmenko, “Experimental Performance of a Miniature Fabry-Perot Fiber Optic Hydrophone,” Proceedings of 8th Optical Fiber Sensors Conference, Monterey, Calif., Jan. 29-31, 1992, pages 354-357.
Photonic crystal slabs (PCSs) are photonic crystal structures having a spatially periodically varying refractive index. A PCS exhibits guided resonance optical modes that are strongly confined within the PCS, but are coupled to incident radiation through a phase matching mechanism due to the periodically varying refractive index. These guided resonance modes are typically manifest in transmission or reflection spectra as sharp Fano lineshapes superimposed on a smoothly varying background. See, e.g. M. Kanskar et al., “Observation of leaky slab modes in an air-bridged semiconductor waveguide with a two-dimensional photonic lattice,” Appl. Phys. Lett., vol. 70, page 1438 (1997); V. N. Astratov et al., “Resonant coupling of near-infrared radiation to photonic band structure waveguides,” J. Lightwave Technol., vol. 17, page 2050 (1999); and S. Fan and J. D. Joannopoulos, “Analysis of guided resonances in photonic crystal slabs,” Phys. Rev. B, vol. 65, page 235112 (2002). Such guided resonance modes have been used previously as optical filters or mirrors in light emitting diodes and lasers.